An internal combustion engine that is generally used as a driving force source of a vehicle has such a characteristic that an output torque increases with an increase in rotation speed, whereas a driving force that is required of a vehicle is generally large at a low vehicle speed and is relatively small at a high vehicle speed. That is, a vehicle requires a torque having an inverse characteristic to a torque based on the output characteristic of the internal combustion engine. In addition, an operating point at which the efficiency of the internal combustion engine is high is limited. Therefore, in a vehicle that uses the internal combustion engine as a driving force source, a transmission that is able to change its speed ratio as needed is mounted, and the speed ratio is set as needed on the basis of a traveling state of the vehicle, such as a vehicle speed and an accelerator operation amount. Thus, a required driving force is obtained, and the internal combustion engine is operated at an operating point having a high efficiency. In addition, when there is a step between speed ratios, if an operating point: having a high thermal efficiency is at a rotation speed that can be set at a speed ratio between those speed ratios, thermal efficiency (or fuel economy) deteriorates in an operating state during a period of a change from one of the speed ratios to the other one of the speed ratios. Therefore, recently, a continuously variable transmission is used instead of a stepped transmission.
A belt-type continuously variable transmission and a toroidal-type continuously variable transmission are widely known as a continuously variable transmission for a vehicle. The former belt-type continuously variable transmission includes a pair of pulleys, each of which increases or reduces the winding radius of a belt by changing the width of a groove in which the belt is wound. The belt-type continuously variable transmission is configured as follows. The winding radius of the belt is reduced by increasing the groove width of one of the pulleys, and the winding radius of the other one of the pulleys is reduced accordingly to cause the winding radius of the belt to increase. The latter toroidal-type continuously variable transmission is configured as follows. Power rollers are sandwiched between a pair of opposed discs. When a line connecting contact points of each power roller with the respective discs is inclined with respect to a rotation central axis of each disc, a difference in rotation speed between the discs arises. As the inclined angle (tilt angle) increases, the difference in rotation speed between the discs, that is, a speed ratio, deviates from “1”.
In these continuously variable transmissions, in order to be able to continuously change the speed ratio, torque is transmitted by utilizing friction force between each pulley and the belt or friction force between each disc and each power roller. Because a friction force is the product of a friction coefficient at a contact point of two members and a normal load (or a load in the normal direction), the normal load is increased with a torque to be transmitted. In the belt-type continuously variable transmission for a vehicle, the normal load is a load by which each pulley clamps the belt. For example, a hydraulic actuator is integrally formed with each pulley, and the load is generated by hydraulic pressure that is supplied to each hydraulic actuator.
On the other, hand, a large driving force is required at the start of the vehicle, whereas a driving force that is required in a steady traveling state, that is, during cruising, is smaller than that at the start of the vehicle. Therefore, it is required to increase the normal load for generating the above-described friction force at the start of the vehicle, and a hydraulic pressure for generating a clamping force is increased at the start of the vehicle in the belt-type continuously variable transmission. If a hydraulic device that generates a large hydraulic pressure, is provided in preparation for a start-moving time that is a relatively short time as a driving state of the vehicle, a driving device and a hydraulic system for the driving device increase in size, and there is a possibility that fuel economy deteriorates as the hydraulic device generates a high hydraulic pressure.
A device that is able to eliminate such an inconvenience is described in Japanese Patent Application Publication No. 2005-308041 (JP 2005-308041 A), Japanese Patent Application Publication No. 2004-076876 (JP 2004-076876 A), Japanese Patent Application Publication No. 2000-130548 (JP 2000-130548 A), or the like. Of these, in the device described in JP 2005-308041 A, power output from an engine is transmitted to a sun gear of a single-pinion-type planetary gear train that constitutes a forward-reverse switching mechanism, and a clutch is provided to couple the sun gear to an input shaft integrated with a primary pulley of the belt-type continuously variable transmission. An input gear is fitted to an outer peripheral side of the input shaft via a one-way clutch. The input gear is coupled to a ring gear in the forward-reverse switching mechanism. The one-way clutch is configured to be engaged when the input shaft rotates at a higher speed than the outer peripheral-side input gear in a forward rotation direction. An output gear is fitted via another one-way clutch to-an outer peripheral side of an output shaft integrated with a secondary pulley. An idle gear is arranged between the input gear and the output gear. The input gear and the output gear are in mesh with the idle gear. That is, the input gear and the output gear both are configured to rotate in the same direction. A gear ratio (speed ratio) between these input gear and output gear is set to a speed ratio slightly smaller than the largest speed ratio that can be set by the continuously variable transmission formed of the pulleys and the belt wound around these pulleys. The other one-way clutch is configured to be engaged when the output shaft rotates at a higher speed than the output gear in the forward rotation direction. A friction clutch is provided in parallel with the other one-way clutch. In addition, in order to set a backward traveling state, a brake that fixes a carrier in the forward-reverse switching mechanism is provided.
Thus, in the device described in JP 2005-308041 A, for example, when the vehicle starts moving in order to travel forward, the sun gear and the input shaft are coupled to each other by the clutch, torque is transmitted via the input shaft to a main shift path mainly formed of the continuously variable transmission, and torque is transmitted by engaging the one-way clutch with an auxiliary shift path mainly formed of the above-described gears. In this case, because the speed ratio of the gear train is slightly smaller than the maximum speed ratio of the continuously variable transmission, the output gear rotates at a higher speed than the output shaft, the output shaft-side one-way clutch enters a released state, and torque is transmitted to drive wheels via the gear train. That is, large torque at the start of the vehicle is not applied to the continuously variable transmission. After the vehicle starts moving, when the speed ratio of the continuously variable transmission is gradually reduced with an increase in vehicle speed, the rotation speed of the output shaft integrated with the secondary pulley reaches the rotation speed of the output gear provided on the outer peripheral side of the output shaft, and the rotation speed of the output shaft further increases as a result of a reduction in speed ratio. As a result, the output shaft-side one-way clutch enters an engaged state, and torque is transmitted to the drive wheels via the continuously variable transmission. In this case, because the input shaft-side one-way clutch enters a released state, no interlocking state occurs.
In the device described in JP 2004-076876 A, a forward-reverse switching mechanism formed of a single-pinion-type planetary gear train is provided between an input shaft that transmits power output from an engine and a primary pulley in a belt-type continuously variable transmission, a ring gear in the forward-reverse switching mechanism is coupled to the primary pulley so as to integrally rotate with the primary pulley, and the input shaft is coupled to a sun gear. Thus, a forward traveling state is established by coupling the sun gear and the ring gear to each other by a clutch, and a backward traveling state is established by fixing a carrier by a brake. In addition, a gear train having a speed ratio larger than the maximum speed ratio of the continuously variable transmission is provided between the input shaft and an output shaft integrated with a secondary pulley, an input gear that constitutes the gear train is integrated with the input shaft, an output gear coupled to the input shaft via an idle gear is rotatably fitted to the output shaft, and a one-way clutch and a friction clutch are arranged in series between the output gear and the output shaft.
Thus, in the case where the vehicle starts moving in the forward traveling state, when the clutch for coupling the input shaft to the primary pulley is released and the output shaft-side clutch is engaged, torque is transmitted from the input shaft to the output shaft via the gear train, the one-way clutch and the clutch arranged in series with these gear train and one-way clutch. When the input shaft and the primary pulley are coupled to each other by the clutch from this state, because the maximum speed ratio of the continuously variable transmission is slightly smaller than the speed ratio of the gear train, the secondary pulley and the output shaft integrated with the secondary pulley rotate at a rotation speed higher than before, more specifically, a higher rotation speed than the output gear, so the one-way clutch is released. That is, torque is transmitted to the output shaft via the continuously variable transmission. In this way, because the gear train transmits torque at the start of the vehicle, large torque at the start of the vehicle is not applied to the continuously variable transmission.
JP 2000-130548 A describes a transmission device configured as in the case of the above-described device described in JP 2004-076876 A. In the transmission device described in JP 2000-130548 A as well, a one-way clutch and a friction clutch are arranged in parallel between an output-side gear in a gear train and an output shaft. The gear train transmits torque at the start of the vehicle. The output shaft is integrated with a secondary pulley.
In the device described in any one of these publications, the gear train is provided in parallel with the belt-type continuously variable transmission, and the device is configured to transmit torque for moving the vehicle mainly via the gear train at the start of the vehicle. In order to transmit torque via the continuously variable transmission in a forward traveling state, the transmission path of torque is changed, and the change is configured to be carried out by using the one-way clutch. However, the torque transmission direction of the one-way clutch is limited to one direction, while torque needs to be transmitted in any one of forward and reverse directions at the time when the vehicle actually travels, and it is required not to cause the one-way clutch to function depending on the configuration of the torque transmission path. Therefore, as described in the above-described publications, it is required to use both the one-way clutch and the friction clutch. Thus, with the configuration described in each of the above-described publications, even when it is possible to avoid or suppress application of large torque to the continuously variable transmission at the start of the vehicle, the configuration of the device as a whole increases in size, so there is a possibility that vehicle mountability is impaired.
The device described in JP 2005-308041A and the device described in JP 2004-076876 A each include the forward-reverse switching mechanism formed of the planetary gear train. However, in the former configuration described in JP 2005-308041 A, when the vehicle travels while transmitting torque by the belt-type continuously Variable transmission, torque from the engine is transmitted to the sun gear and torque from the gear train is transmitted to the ring gear. Therefore, there occurs a large rotation speed difference between the sun gear, pinion gears and the ring gear, and there is a possibility that this causes a loss of power, degradation of lubricating oil, noise or vibrations. In the latter configuration described in JP 2004-076876 A, when the vehicle travels while the gear train is transmitting torque, torque from the engine is transmitted to the sun gear of the planetary gear train that constitutes the forward-reverse switching mechanism, and torque is transmitted from the output shaft side to the ring gear via the continuously variable transmission. As a result, as in the case of the device described in JP 2005-308041 A, there occurs a large rotation speed difference between the sun gear, the pinion gears and the ring gear, and there is a possibility that this causes a loss of power, degradation of lubricating oil, noise or vibrations.